Universitätsklinikum Würzburg, Medizinische Klinik und Poliklinik I, Oberdürrbacher Str. 6, 97080 Würzburg, Germany.
Universitätsklinikum Würzburg, Medizinische Klinik und Poliklinik I, Oberdürrbacher Str. 6, 97080 Würzburg, Germany.
Life Sci. 2020 May 1;248:117474. doi: 10.1016/j.lfs.2020.117474. Epub 2020 Feb 27.
BACKGROUND/OBJECTIVES: Nicotinamide N-methyltransferase (NNMT) is a novel regulator of energy homeostasis in adipocytes. NNMT expression in adipose tissue is increased in obesity and diabetes. Knockdown of NNMT prevents mice from developing diet-induced obesity, which is closely linked to insulin resistance. An early sign of systemic insulin resistance is reduced expression of glucose transporter 4 (GLUT4) selectively in adipose tissue. Adipose tissue-specific knockout and overexpression of GLUT4 cause reciprocal changes in NNMT expression. The aim of the current study was to elucidate the mechanism that regulates NNMT expression in adipocytes.
3T3-L1 adipocytes were cultured in media with varying glucose concentrations or activators and inhibitors of intracellular pathways. NNMT mRNA and protein levels were measured with quantitative polymerase chain reaction and Western blotting.
Glucose deprivation of 3T3-L1 adipocytes induced a 2-fold increase in NNMT mRNA and protein expression. This effect was mimicked by inhibition of glucose transport with phloretin, and by inhibition of glycolysis with the phosphoglucose isomerase inhibitor 2-deoxyglucose. Conversely, inhibition of the pentose phosphate pathway did not affect NNMT expression. Pharmacological activation of the cellular energy sensor AMP-activated protein kinase (AMPK) and inhibition of the mammalian target of rapamycin (mTOR) pathway caused an increase in NNMT levels that was similar to the effect of glucose deprivation. Activation of mTOR with MHY1485 prevented the effect of glucose deprivation on NNMT expression. Furthermore, upregulation of NNMT levels depended on functional autophagy and protein translation.
Glucose availability regulates NNMT expression via an mTOR-dependent mechanism.
背景/目的：烟酰胺 N-甲基转移酶（NNMT）是脂肪细胞能量稳态的新型调节因子。脂肪组织中 NNMT 的表达在肥胖和糖尿病中增加。NNMT 的敲低可防止小鼠发生饮食诱导的肥胖，而饮食诱导的肥胖与胰岛素抵抗密切相关。全身胰岛素抵抗的早期迹象是葡萄糖转运蛋白 4（GLUT4）在脂肪组织中的选择性表达降低。脂肪组织特异性敲除和过表达 GLUT4 导致 NNMT 表达的相互变化。本研究旨在阐明调节脂肪细胞中 NNMT 表达的机制。
在含有不同葡萄糖浓度的培养基或细胞内途径的激活剂和抑制剂中培养 3T3-L1 脂肪细胞。用定量聚合酶链反应和 Western blot 法测量 NNMT mRNA 和蛋白水平。
3T3-L1 脂肪细胞的葡萄糖剥夺诱导 NNMT mRNA 和蛋白表达增加 2 倍。该作用可被 phloretin 抑制葡萄糖转运所模拟，也可被磷酸葡萄糖异构酶抑制剂 2-脱氧葡萄糖抑制糖酵解所模拟。相反，戊糖磷酸途径的抑制不影响 NNMT 表达。细胞能量感受器 AMP 激活蛋白激酶（AMPK）的药理学激活和哺乳动物雷帕霉素靶蛋白（mTOR）途径的抑制导致 NNMT 水平增加，与葡萄糖剥夺的作用相似。用 MHY1485 激活 mTOR 可防止葡萄糖剥夺对 NNMT 表达的影响。此外，NNMT 水平的上调依赖于功能性自噬和蛋白质翻译。
葡萄糖的可用性通过 mTOR 依赖的机制调节 NNMT 的表达。
